wane (moon)
虧 centers on the moon waning or diminishing, describing a phase where its visible shape decreases.
虧 is a phono-semantic compound: the left side 雐 likely contributes the sound, while the right side 亏 (originally depicting breath or loss) suggests the idea of diminishing or waning. The exact historical development is uncertain.
The left side looks like a bird (隹) under a tiger's stripes (虍), and the right side 亏 resembles a breath escaping. Imagine a bird watching the moon's breath fade away as it wanes.
For キ, picture a key (キー) that unlocks the mystery of the waning moon: key -> キ, and the moon's light diminishes as you turn it.